1. Field of the Invention
The present invention relates to a magnetic disk device.
2. Description of the Related Art
In recent years, a magnetic disk device (hard disk drive (HDD)) is used in a large computer, a personal computer (PC), a home electric appliance, a car-mounted navigation system, an information terminal, a mobile phone, and the like. The maximum read/write (R/W) bandwidth in the magnetic disk device is proportional to the number of rotations (rotation speed) of the magnetic disk device and a track density of a disk platter that stores therein data. Specifically, the maximum R/W bandwidth in the magnetic disk device corresponds to a value that is obtained by multiplying a bit density along one track by the number of rotations. For example, in a case of a 1.8-inch HDD spinning at 4200 revolutions per minute (rpm), because the HDD has about 900 sectors, each of which holds 512 bytes (B), in one track, 460 kilobytes (KB) data is recorded in one track. If the HDD rotates at 4200 rpm, data can be read/written at the maximum speed of 32 MB/s (=460 KB×4200/60).
In such HDD, the recording density (bit per inch (bpi)), i.e., data amount written in one inch along a track, is the same between an inner track and an outer track. However, because the inner track has a smaller circumference than the outer track, the number of zones each including a plurality of sectors (the number of zones per one round) is smaller in the inner track than in the outer track. Because the number of rotations is the same between the inner track and the outer track, the maximum bandwidth becomes smaller in the inner track than the outer track. Therefore, the maximum bandwidth of the HDD becomes large as the HDD is increased to 1.8 inches, 2.5 inches, and 3.5 inches in size and as the number of rotations of the HDD is increased to 4200 rpm, 5200 rpm, and 7200 rpm (for example, refer to “Hard Disk Drive Mechatronics and Control” CRC Press ISBN-10:0-8493-7253-4 by Abdullah Al Mamun et al.). In the HDD, if data can be read/written with the maximum bandwidth, the data can be read/written on a disk in a short time.
However, in an operating system (OS), such as Windows (registered trademark) XP, Windows (registered trademark) Vista, and Linux (registered trademark), or the like, the minimum unit of file writing is 4 KB (64 KB at the maximum) in a file format, so that small files are often read/written randomly in a PC. If data is randomly read/written, when data is read/written on a track accompanying a long rotation of the disk over different sectors or in the same sector (movement between tracks over a long distance), the effective R/W bandwidth degrades extremely with respect to the maximum bandwidth. For example, in 1.8 HDD, the seek time is 3 milliseconds (ms) for moving to an adjacent track and is 22 ms for moving from the inner track to the outer track. The average seek time in 1.8 HDD is about 11 ms.
For performing the read/write operation in the same track, the seek time to the sector next to the sector on which the read/write operation was performed is 0 second (s). However, when the disk is rotated half a turn, time loss of 7 ms (=1/(4200/60)/2) occurs. Furthermore, in the worse case in which the read/write operation is performed in a sector of one track before, time loss of 14 ms occurs. For example, when the sum of the seek time and the rotational latency is 8 ms for each 4 KB, the effective R/W bandwidth is 490 KB/s (4 KB/(4 KB/(32 MB/s)+8 ms), so that the performance of the effective R/W bandwidth is dropped to 1/65 of the maximum R/W bandwidth.
As described above, in the conventional technique, when random reading/writing of files of a small size occurs by the operation of the OS or the like, the effective R/W bandwidth degrades substantially from the maximum R/W bandwidth, so that the performance of the read/write operation degrades.